1. Field of the Invention
The present invention relates to cavity prevention in preforming plastic optical fiber, and more particularly, to a cavity-preventing type reactor and a method for fabricating a preform for a plastic optical fiber using the same.
2. Description of the Related Art
Optical fibers used in the field of telecommunications are generally classified into a single-mode fiber and a multi-mode fiber in terms of the transmission mode of optical signal. Optical fibers currently used for long distance, high speed communications are mostly step-index, single-mode optical fibers based on quartz glass. These optical fibers have a diameter as small as 5 microns to 10 microns, and as a result, use of these glass optical fibers creates significant difficulties in terms of achieving proper alignment and connection. Accordingly, these glass optical fibers are associated with significant expenses relating to achieving proper alignment and connection.
Multi-mode glass optical fibers have a diameter that is larger than that of single-mode optical fibers and may be used for short distance communications such as in local area networks (LANs). However, these multi-mode glass optical fibers, in addition to being fragile, also suffer from expensive costs relating to achieving proper alignment and connection and therefore are not widely used. Accordingly, these multi-mode glass optical fibers have been mainly used for short distance communication applications of up to 200 meters, such as in LANs, which normally use a metal cable, for example, a twisted pair or coaxial cable. Since the data transmission capacity or bandwidth of the metal cable may be as low as about 150 Mbps, it cannot satisfy standards for transmission capacity, such as a speed of 625 Mbps that is associated with modem asynchronous transfer mode (ATM) for data transmission.
To overcome the foregoing, plastic optical fibers, which can be used in short distance communication applications, such as LANs, have been developed. The diameter of plastic optical fibers may be as large as 0.5 to 1.0 mm which is 100 or more times larger than that of glass optical fibers. Due to the flexibility of plastic optical fibers, proper alignment and connection are much easier to achieve with plastic optical fibers than with glass optical fibers. Moreover, since polymer-based connectors may be inexpensively produced using a compression molding, these connectors may be used for both alignment and connection, thereby further reducing costs.
Plastic optical fibers may have a step-index (SI) structure, in which a refractive index changes stepwise in a radial direction, or a graded-index (GI) structure, in which a refractive index changes gradually in a radial direction. However, since plastic optical fibers having a SI structure are characterized by a high modal dispersion, the transmission capacity (or bandwidth) of a signal cannot be larger than that of cable. On the other hand, since plastic optical fibers having a GI structure are characterized by a low modal dispersion, it can have a large transmission capacity. Therefore, GI plastic optical fibers have become widely used as a communication medium for short distance, high-speed communications.
Conventional methods for fabricating GI plastic optical fiber are mainly classified into two methods. A first method comprises a batch process, wherein a preliminary cylindrical molding product, namely, a preform in which a refractive index changes in a radial direction, is fabricated, and then the resultant preform is heated and drawn to fabricate GI plastic optical fiber. A second method comprises a continuous process wherein a plastic fiber is produced by an extrusion process, and then a low molecular material contained in the fiber is extracted to obtain GI plastic optical fiber. Alternatively, a low molecular material can be introduced to the fiber in a radial direction.
The first method can be used successfully to fabricate a GI plastic optical fiber having a data transmission capacity of 2.5 Gbps, and the second method can be used successfully to fabricate a plastic optical fiber having a relatively large data transmission capacity.
Another conventional method for fabricating GI preforms employs very high rotational speeds, e.g., about 20,000 rpm). This method uses the principle that if a mixture of monomers or polymer-dissolving monomers having different densities and refractive indexes is polymerized in a very strong centrifugal field over 10,000×d−0.5 rpm, where d is a diameter of a preform, a concentration gradient is generated on account of a density gradient, and thereby, a refractive index gradient is generated. While a high rotational speed is known in the art as being advantageous in producing a definite refractive index profile, even in a relatively weak centrifugal field, a concentration (or refractive index) gradient develops, if there is a density difference between the components of a mixture.
A significant disadvantage of the aforementioned methods is a problem caused by volume shrinkage that occurs during (radical) chain polymerization, which is common in the fabrication of GI preform. For example, the extent of volume shrinkage from methylmethacrylate to poly(methylmethacrylate) is over 20%. Since volume shrinkage occurs when monomers are polymerized (to produce a polymer), a preform for a plastic optical fiber fabricated under the rotation of a reactor forms a central cavity in the shape of a tube. Thus, it is required to fill the cavity with additional monomer, prepolymer or polymer-dissolving monomers in order to fabricate a cavity-free preform.
Accordingly, when a plastic optical fiber is fabricated using a cavity-filling type preform, the probability of developing a discontinuity of the refractive index profile increases in proportion to the size of a cavity. This discontinuity can lead to a significant scattering at the interface of the plastic optical fiber. Such scattering may reduce the data transmission capacity of the plastic optical fiber to such a degree that the optical fiber may not be useable at all.
Furthermore, in the process of filling the cavity, the quality of the resultant preform may deteriorate due to contact with minute particles of dust, air and/or moisture. Thus, additional appliance and manufacturing expense may be required in order to prevent this contact and/or to compensate for this degradation.